Many electrical systems are modular so that an electrical system comprises modules providing the functionality of the electrical system and a frame structure for mechanically supporting the modules and for connecting the modules communicatively to each other. An electrical system of the kind described above can be, for example but not necessarily, telecommunication equipment where the modules can be for example plug-in units of the telecommunication equipment and the frame structure can be a rack of the telecommunication equipment. Each of the above-mentioned modules comprises typically a circuit board, one or more electrical components on the circuit board, and one or more cooling elements in heat conductive relation with one or more of the electrical components. Each cooling element comprises typically cooling fins and/or tubular cooling channels. A cooling element is needed when heat generation of an electrical component is so high with respect to the mechanical dimensions of the electrical component that the ability of the outer surface of the electrical component to transfer heat to the ambient air is not sufficient. A cooling element is typically used with a high performance integrated circuit that can be for example a processor.
The mechanical arrangement of a modular electrical system of the kind described above can be such that cooling air meets first a cooling element that is cooling one or more electrical components of a first module and subsequently the same cooling air meets another cooling element that is cooling one or more electrical components of a second module. As evident, the cooling of the electrical components of the second module is not as effective as the cooling of the electrical components of the first module because the cooling air is warmed when cooling the electrical components of the first module and the electrical components of the second module are cooled by the warmed cooling air. A situation of the kind described above takes place for example in telecommunication equipment where the cooling air flows vertically and two or more plug-in units are mounted successively in the vertical direction so that the circuit boards of these plug-in units are co-planar, i.e. the circuit boards are in the same geometric plane.